A barre is a chord where one finger (usually the index finger) presses down multiple strings at once across a fret — it replaces the nut and makes every open CAGED shape playable anywhere on the fretboard. That’s exactly why it hurts at first: the index finger needs strength and the right technique, which only develops over time.
Why it doesn’t work at first
The most common mistakes: the finger sits too far from the fret (needs more force), it presses with the flat pad instead of the slightly angled edge, or the thumb sits too high instead of behind the neck as a counter-anchor.
Training path: start small
Instead of forcing a full barre across all six strings right away, start by cleanly muting just the top two or three strings (e.g. a reduced F major shape). The necessary finger strength builds up over days and weeks — regular short practice gets you further here than rare, long sessions of forcing it.